Gas-phase polymerization processes are economical processes for the polymerization of C2-C8-olefins. Such gas-phase polymerization processes can, in particular, be carried out as gas-phase fluidized-bed processes in which the polymer particles are kept in suspension by a suitable gas stream. Processes of this type are described, for example, in EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826.
During operation of the polymerization reactor, fine particles can be discharged from the reactor by the gas or can fall back into the fluidized bed. However, fine particles also tend to deposit and accumulate on the interior wall of the reactor, particularly in the calming zone known as the freeboard. This can result in formation of deposits which, when they reach a relatively great thickness, become detached and can, for example, block the polymer discharge facility or the distributor plate, while smaller agglomerates are discharged from the reactor together with the product and impair the quality of the latter.
The polymerization reaction is carried out using a catalyst and, if appropriate, a cocatalyst. While the preparation of polyolefins having a relatively high molecular weight and thus a melt flow rate (MFR) at 2.16 kg and 190° C. in accordance with DIN EN ISO 1133 of less than 4 g/10 min is unproblematical in process engineering terms when using Ziegler-Natta catalysts or chromium catalysts, the formation of fine particles presents considerable problems when using catalysts comprising organic transition metal compounds, in particular metallocene catalysts, especially when starting up the reactor. Many of these catalysts, in particular metallocene catalysts based on bis(1-methyl-3-butylcyclopentadienyl)zirkonium dichloride, tend to form fine particles during start-up of the reactor and these accumulate in the calming zone, form deposits and lumps and can hinder the start-up process to such an extent that it has to be terminated.
Various approaches have hitherto been described for improving the start-up behavior and operation of gas-phase fluidized-bed reactors. Thus, WO 99/03901 and EP 692 500 A1 use soundwaves to prevent adhesion of particles, particularly at low heights of the fluidized bed in the reactor.
In WO 99/00430, the gas stream taken off from the reactor is divided into two parts and one part is cooled in a condenser to below the dew point of the gas and fed directly back into the reactor in an amount of at least 10 l per cubic meter of fluidized bed per hour and the other part is conveyed in a bypass around the condenser, passed through a heat exchanger and then fed back into the reactor.
However, all approaches employed hitherto are associated with a relatively great outlay in terms of apparatus and/or give unsatisfactory results when a gas-phase fluidized-bed reactor is operated using catalysts based on organic transition metal compounds.